If further elements with higher atomic numbers than this are discovered, they will be placed in additional periods, laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements concerned. Any additional periods are expected to contain a larger number of elements than the seventh period, as they are calculated to have an additional so-called g-block, containing at least 18 elements with partially filled g-orbitals in each period. An eight-period table containing this block was suggested by Glenn T. Seaborg in 1969.[1][2] The first element of the g-block may have atomic number 121, and thus would have the systematic nameunbiunium. Despite many searches, no elements in this region have been synthesized or discovered in nature.[3]

According to the orbital approximation in quantum mechanical descriptions of atomic structure, the g-block would correspond to elements with partially filled g-orbitals, but spin-orbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number. While Seaborg's version of the extended period had the heavier elements following the pattern set by lighter elements, as it did not take into account relativistic effects, models that take relativistic effects into account do not. Pekka Pyykkö and Burkhard Fricke used computer modeling to calculate the positions of elements up to Z = 172, and found that several were displaced from the Madelung rule.[4][5] As a result of uncertainty and variability in predictions of chemical and physical properties of elements beyond 120, there is currently no consensus on their placement in the extended periodic table.

As early as 1940, it was noted that a simplistic interpretation of the relativisticDirac equation runs into problems with electron orbitals at Z > 1/α ≈ 137, suggesting that neutral atoms cannot exist beyond element 137, and that a periodic table of elements based on electron orbitals therefore breaks down at this point.[7] On the other hand, a more rigorous analysis calculates the analogous limit to be Z ≈ 173 where the 1s subshell dives into the Dirac sea, and that it is instead not neutral atoms that cannot exist beyond element 173, but bare nuclei, thus posing no obstacle to the further extension of the periodic system. Atoms beyond this critical atomic number are called supercritical atoms.

Contents

Heavier elements beyond the actinides were first proposed to exist as early as 1895, when the Danish chemist Hans Peter Jørgen Julius Thomsen predicted that thorium and uranium formed part of a 32-element period which would end at a chemically inactive element with atomic weight 292 (not far from the 294 known today for the first and only discovered isotope of oganesson). In 1913, the Swedish physicist Johannes Rydberg similarly predicted that the next noble gas after radon would have atomic number 118, and purely formally derived even heavier congeners of radon at Z = 168, 218, 290, 362, and 460, exactly where the Aufbau principle would predict them to be. Niels Bohr predicted in 1922 the electronic structure of this next noble gas at Z = 118, and suggested that the reason why elements beyond uranium were not seen in nature was because they were too unstable. The German physicist and engineer Richard Swinne published a review paper in 1926 containing predictions on the transuranic elements (he may have coined the term) in which he anticipated modern predictions of an island of stability: he had hypothesised since 1914 that half-lives should not decrease strictly with atomic number, but suggested instead that there might be some longer-lived elements at Z = 98–102 and Z = 108–110, and speculated that such elements might exist in the core, in iron meteorites, or in the ice caps of Greenland where they had been locked up from their supposed cosmic origin.[8] By 1955, these elements were called superheavy elements.[9]

The first predictions on properties of undiscovered superheavy elements were made in 1957, when the concept of nuclear shells was first explored and an island of stability was theorized to exist around element 126.[10] In 1967, more rigorous calculations were performed, and the island of stability was theorized to be centered at the then-undiscovered flerovium (element 114); this and other subsequent studies motivated many researchers to search for superheavy elements in nature or attempt to synthesize them at accelerators.[9] Many searches for superheavy elements were conducted in the 1970s, all with negative results. As of December 2018[update], synthesis has been attempted for every element up to and including unbiseptium (Z = 127), except unbitrium (Z = 123),[11][12][13] with the heaviest successfully synthesized element being oganesson in 2002 and the most recent discovery being that of tennessine in 2010.[11]

As some superheavy elements were predicted to lie beyond the seven-period periodic table, an additional eighth period containing these elements was first proposed by Glenn T. Seaborg in 1969. This model continued the pattern in established elements and introduced a new g-block and superactinide series beginning at element 121, raising the number of elements in period 8 compared to known periods.[1][2][9] These early calculations failed to consider relativistic effects that break down periodic trends and render simple extrapolation impossible, however. In 1971, Fricke calculated the periodic table up to Z = 172, and discovered that some elements indeed had different properties that break the established pattern,[4] and a 2010 calculation by Pekka Pyykkö also noted that several elements might behave differently than expected.[14] It is unknown how far the periodic table might extend beyond the known 118 elements, as heavier elements are predicted to be increasingly unstable. Glenn T. Seaborg suggested that the highest possible element may be under Z = 130,[15] while Walter Greiner predicted that there may not be a highest possible element.

There is currently no consensus on the placement of elements beyond atomic number 120 in the periodic table. The table below shows one possibility for the appearance of the eighth period, with placement of elements primarily based on their predicted chemistry.[16][17]

All of these hypothetical undiscovered elements are named by the International Union of Pure and Applied Chemistry (IUPAC) systematic element name standard which creates a generic name for use until the element has been discovered, confirmed, and an official name approved. These names are typically not used in the literature, and are referred to by their atomic numbers; hence, element 164 would usually not be called "unhexquadium" (the IUPAC systematic name), but rather "element 164" with symbol "164", "(164)", or "E164".[18]

At element 118, the orbitals 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, 5f, 6s, 6p, 6d, 7s and 7p are assumed to be filled, with the remaining orbitals unfilled. A simple extrapolation from the Aufbau principle would predict the eighth row to fill orbitals in the order 8s, 5g, 6f, 7d, 8p; but after element 120, the proximity of the electron shells makes placement in a simple table problematic. Although a simple extrapolation of the periodic table, following Seaborg's original concept, would put the elements after 120 as follows: 121-138 form the g-block superactinides; 139-152 form the f-block superactinides, 153-161 would be transition metals; 162-166 post-transition metals; 167=halogen; 168=noble gas; 169=alkali metal; 170=alkaline earth metal, Dirac-Fock calculations predict that it will most likely go: 121-142 form the g-block superactinides; 143-156 form the f-block superactinides; 157-165 form the transition metals; 166-170 post-transition metals; 171=halogen; 172=noble gas.

Not all models show the higher elements following the pattern established by lighter elements. Pekka Pyykkö, for example, used computer modeling to calculate the positions of elements up to Z = 172 and their possible chemical properties. He found that several elements were displaced from the Madelung energy-ordering rule as a result of overlapping orbitals; this is caused by the increasing role of relativistic effects in heavy elements.[5][14] He predicts that the orbitals will fill up in this order: 8s, 5g, the first two spaces of 8p, 6f, 7d, 9s, the first two spaces of 9p, the rest of 8p. This corresponds to elements 119 and 120 being alkali and alkaline earth metals, 121-138 g-block superactinides, 139 and 140 post-transition metals placed below thallium and lead, respectively, 141-154 f-block superactinides, 155-164 transition metals, 165 and 166 alkali and alkaline earth metals below 119 and 120 and opening period 9, 167-168 below 139 and 140, and 169-172 completing period 8. He also suggests that period 8 be split into three parts: 8a, containing 8s, 8b, containing the first two elements of 8p, and 8c, containing 7d and the rest of 8p.[14]

Fricke's predictions — up to element 184 — also found some elements to be displaced from the Aufbau principle as a consequence of relativistic effects.[4][19] He predicted that after element 120, a long transition series (the superactinides) characterized by the filling of 5g and 6f orbitals would begin, and continue until element 154. A fifth transition metal series in which 7d orbitals are filled then would contain elements 155-164, and the eighth period would possibly end there.[19] It is also possible that element 157 is in fact the first 7d transition metal, another shift resulting from relativistic effects. Elements 165 and 166 were predicted to be alkali and alkaline earth metals, though they may also mix characteristics of groups 11 and 12 and instead be placed below roentgenium and copernicium, respectively. Finally, elements 167 to 172 would be the heaviest members of groups 13-18.[19] Fricke initially proposed that elements 165-172 would constitute a ninth period, as these elements may follow a similar pattern to periods 2 and 3 and have no transition metals. Due to overlapping orbitals and possible mixing of group characteristics in superheavy elements, element 172 may instead close period 8, however.[19]

The synthesis of ununennium was first attempted in 1985 by bombarding a target of einsteinium-254 with calcium-48 ions at the superHILAC accelerator at Berkeley, California:

25499Es + 4820Ca → 302119Uue* → no atoms

No atoms were identified, leading to a limiting cross section of 300 nb.[20] Later calculations suggest that the cross section of the 3n reaction (which would result in 299Uue and three neutrons as products) would actually be six hundred thousand times lower than this upper bound, at 0.5 pb.[21]

As ununennium is the lightest undiscovered element, it has been the target of synthesis experiments by both German and Russian teams in recent years.[22][23] The Russian experiments were conducted in 2011, and no results were released, strongly implying that no ununennium atoms were identified. From April to September 2012, an attempt to synthesize the isotopes 295Uue and 296Uue was made by bombarding a target of berkelium-249 with titanium-50 at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany.[24][25] Based on the theoretically predicted cross-section, it was expected that an ununennium atom would be synthesized within five months of the beginning of the experiment.[26]

24997Bk + 5022Ti → 299119Uue* → 296119Uue + 3 10n

24997Bk + 5022Ti → 299119Uue* → 295119Uue + 4 10n

The experiment was originally planned to continue to November 2012,[27] but was stopped early to make use of the 249Bk target to confirm the synthesis of tennessine (thus changing the projectiles to 48Ca).[28] This reaction between 249Bk and 50Ti was predicted to be the most favorable practical reaction for formation of ununennium,[25] as it is rather asymmetrical,[26] though also somewhat cold.[28] (The reaction between 254Es and 48Ca would be superior, but preparing milligram quantities of 254Es for a target is difficult.)[26] Nevertheless, the necessary change from the "silver bullet" 48Ca to 50Ti divides the expected yield of ununennium by about twenty, as the yield is strongly dependent on the asymmetry of the fusion reaction.[26]

Due to the predicted short half-lives, the GSI team used new "fast" electronics capable of registering decay events within microseconds.[25] No ununennium atoms were identified, implying a limiting cross-section of 70 fb.[28] The predicted actual cross-section is around 40 fb, which is at the limits of current technology.[26]

The team at the Joint Institute for Nuclear Research in Dubna, Russia, planned to begin experiments on the synthesis of ununennium and unbinilium using the 249Bk+50Ti and 249Cf+50Ti reactions in 2019 using a new experimental complex.[29][30] The team at RIKEN in Japan also planned to make attempts on these elements starting in 2018 with 248Cm targets using the 248Cm+51V[31] and 248Cm+54Cr reactions.[32] The former has been underway since June 2018.[31]

No atoms were detected providing a limit of 1.6 pb for the cross section at the energy provided. The GSI repeated the experiment with higher sensitivity in three separate runs in April–May 2007, January–March 2008, and September–October 2008, all with negative results, reaching a cross section limit of 90 fb.[38]

In June–July 2010, and again in 2011, after upgrading their equipment to allow the use of more radioactive targets, scientists at the GSI attempted the more asymmetrical fusion reaction:[39]

24896Cm + 5424Cr → 302120Ubn* → no atoms

It was expected that the change in reaction would quintuple the probability of synthesizing unbinilium,[40] as the yield of such reactions is strongly dependent on their asymmetry.[26] Three correlated signals were observed that matched the predicted alpha decay energies of 299Ubn and its daughter295Og, as well as the experimentally known decay energy of its granddaughter 291Lv. However, the lifetimes of these possible decays were much longer than expected, and the results could not be confirmed.[41][42][39]

In August–October 2011, a different team at the GSI using the TASCA facility tried a new, even more asymmetrical reaction:[43][28]

24998Cf + 5022Ti → 299120Ubn* → no atoms

Because of its asymmetry,[44] the reaction between 249Cf and 50Ti was predicted to be the most favorable practical reaction for synthesizing unbinilium, although it is also somewhat cold. No unbinilium atoms were identified, implying a limiting cross-section of 200 fb.[28] Jens Volker Kratz predicted the actual maximum cross-section for producing unbinilium by any of these reactions to be around 0.1 fb;[45] in comparison, the world record for the smallest cross section of a successful reaction was 30 fb for the reaction 209Bi(70Zn,n)278Nh,[26] and Kratz predicted a maximum cross-section of 20 fb for producing the neighbouring ununennium.[45] If these predictions are accurate, then synthesizing ununennium would be at the limits of current technology, and synthesizing unbinilium would require new methods.[45]

The team at the Joint Institute for Nuclear Research in Dubna, Russia, is planning to begin new experiments on the synthesis of ununennium and unbinilium using the 249Bk+50Ti and 249Cf+50Ti reactions in 2019 using a new experimental complex.[29][30] The team at RIKEN in Japan also plans to make attempts on these elements around the same time with 248Cm targets using the 248Cm+51V and 248Cm+54Cr reactions.[32]

These experiments were motivated by early predictions on the existence of an island of stability at N = 184 and Z > 120. No atoms were detected and a yield limit of 5 nb (5,000 pb) was measured. Current results (see flerovium) have shown that the sensitivity of these experiments were too low by at least 3 orders of magnitude.[13]

These results indicate that the synthesis of such heavier elements remains a significant challenge and further improvements of beam intensity and experimental efficiency is required. The sensitivity should be increased to 1 fb in the future for more quality results.

Another unsuccessful attempt to synthesize unbibium was carried out in 1978 at the GSI Helmholtz Center, where a natural erbium target was bombarded with xenon-136 ions:[11]

nat68Er + 13654Xe → 298,300,302,303,304,306Ubb* → no atoms

In particular, the reaction between 170Er and 136Xe was expected to yield alpha-emitters with half-lives of microseconds that would decay down to isotopes of flerovium with half-lives perhaps increasing up to several hours, as flerovium is predicted to lie near the center of the island of stability. After twelve hours of irradiation, nothing was found in this reaction. Following a similar unsuccessful attempt to synthesize unbiunium from 238U and 65Cu, it was concluded that half-lives of superheavy nuclei must be less than one microsecond or the cross sections are very small.[46] More recent research into synthesis of superheavy elements suggests that both conclusions are true.[26][47] The two attempts in the 1970s to synthesize unbibium were both propelled by the research investigating whether superheavy elements could potentially be naturally occurring.[11]

Several experiments studying the fission characteristics of various superheavy compound nuclei such as 306Ubb were performed between 2000–2004 at the Flerov Laboratory of Nuclear Reactions. Two nuclear reactions were used, namely 248Cm + 58Fe and 242Pu + 64Ni.[11] The results reveal how superheavy nuclei fission predominantly by expelling closed shell nuclei such as 132Sn (Z=50, N=82). It was also found that the yield for the fusion-fission pathway was similar between 48Ca and 58Fe projectiles, suggesting a possible future use of 58Fe projectiles in superheavy element formation.[48]

Scientists at GANIL (Grand Accélérateur National d'Ions Lourds) attempted to measure the direct and delayed fission of compound nuclei of elements with Z = 114, 120, and 124 in order to probe shell effects in this region and to pinpoint the next spherical proton shell. This is because having complete nuclear shells (or, equivalently, having a magic number of protons or neutrons) would confer more stability on the nuclei of such superheavy elements, thus moving closer to the island of stability. In 2006, with full results published in 2008, the team provided results from a reaction involving the bombardment of a natural germanium target with uranium ions:

23892U + nat32Ge → 308,310,311,312,314Ubq* → fission

The team reported that they had been able to identify compound nuclei fissioning with half-lives > 10−18 s. This result suggests a strong stabilizing effect at Z = 124 and points to the next proton shell at Z > 120, not at Z = 114 as previously thought. A compound nucleus is a loose combination of nucleons that have not arranged themselves into nuclear shells yet. It has no internal structure and is held together only by the collision forces between the target and projectile nuclei. It is estimated that it requires around 10−14 s for the nucleons to arrange themselves into nuclear shells, at which point the compound nucleus becomes a nuclide, and this number is used by IUPAC as the minimum half-life a claimed isotope must have to potentially be recognised as being discovered. Thus, the GANIL experiments do not count as a discovery of element 124.[11]

The fission of the compound nucleus 312124 was also studied in 2006 at the tandem ALPI heavy-ion accelerator at the Laboratori Nazionali di Legnaro (Legnaro National Laboratories) in Italy:[49]

23290Th + 8034Se → 312Ubq* → fission

Similarly to previous experiments conducted at the JINR (Joint Institute for Nuclear Research), fission fragments clustered around doubly magic nuclei such as 132Sn (Z = 50, N = 82), revealing a tendency for superheavy nuclei to expel such doubly magic nuclei in fission.[48] The average number of neutrons per fission from the 312124 compound nucleus (relative to lighter systems) was also found to increase, confirming that the trend of heavier nuclei emitting more neutrons during fission continues into the superheavy mass region.[49]

The first and only attempt to synthesize unbipentium was conducted in Dubna in 1970–1971 using zinc ions and an americium-243 target:[13]

24395Am + 66,6830Zn → 309,311Ubp* → no atoms

No atoms were detected, and a cross section limit of 5 nb was determined. This experiment was motivated by the possibility of greater stability for nuclei around Z ~ 126 and N ~ 184,[13] though more recent research suggests the island of stability may instead lie at a lower atomic number (such as copernicium, Z = 112), and the synthesis of heavier elements such as unbipentium will require more sensitive experiments.[26]

The first and only attempt to synthesize unbihexium, which was unsuccessful, was performed in 1971 at CERN (European Organization for Nuclear Research) by René Bimbot and John M. Alexander using the hot fusion reaction:[11]

23290Th + 8436Kr → 316126Ubh* → no atoms

High energy (13-15 MeV) alpha particles were observed and taken as possible evidence for the synthesis of unbihexium. Subsequent unsuccessful experiments with higher sensitivity suggest that the 10 mb sensitivity of this experiment was too low; hence, the formation of unbihexium nuclei in this reaction is highly unlikely.[9]

The first and only attempt to synthesize unbiseptium, which was unsuccessful, was performed in 1978 at the UNILAC accelerator at the GSI Helmholtz Center, where a natural tantalum target was bombarded with xenon-136 ions:[11]

A study in 1976 by a group of American researchers from several universities proposed that primordial superheavy elements, mainly livermorium, unbiquadium, unbihexium, and unbiseptium, could be a cause of unexplained radiation damage (particularly radiohalos) in minerals.[9] This prompted many researchers to search for them in nature from 1976 to 1983. A group led by Tom Cahill, a professor at the University of California at Davis, claimed in 1976 that they had detected alpha particles and X-rays with the right energies to cause the damage observed, supporting the presence of these elements. In particular, the presence of long-lived (on the order of 109 years) unbiquadium and unbihexium nuclei, along with their decay products, at an abundance of 10−11 relative to their possible congenersuranium and plutonium, was conjectured.[50] Others claimed that none had been detected, and questioned the proposed characteristics of primordial superheavy nuclei.[9] In particular, they cited that any such superheavy nuclei must have a closed neutron shell at N = 184 or N = 228, and this necessary condition for enhanced stability only exists in neutron deficient isotopes of livermorium or neutron rich isotopes of the other elements that would not be beta-stable[9] unlike most naturally occurring isotopes.[51] This activity was also proposed to be caused by nuclear transmutations in natural cerium, raising further ambiguity upon this claimed observation of superheavy elements.[9]

A criticism of the technique, previously used in purportedly identifying lighter thorium isotopes by mass spectrometry,[54] was published in Physical Review C in 2008.[55] A rebuttal by the Marinov group was published in Physical Review C after the published comment.[56]

A repeat of the thorium-experiment using the superior method of Accelerator Mass Spectrometry (AMS) failed to confirm the results, despite a 100-fold better sensitivity.[57] This result throws considerable doubt on the results of the Marinov collaboration with regard to their claims of long-lived isotopes of thorium,[54]roentgenium[58] and unbibium.[52] It is still possible that traces of unbibium might only exist in some thorium samples, although this is unlikely.[11]

The possible extent of primordial superheavy elements on Earth today is uncertain. Even if they are confirmed to have caused the radiation damage long ago, they might now have decayed to mere traces, or even be completely gone.[59] It is also uncertain if such superheavy nuclei may be produced naturally at all, as spontaneous fission is expected to terminate the r-process responsible for heavy element formation between mass number 270 and 290, well before elements heavier than unbinilium may be formed.[60]

Element 118, oganesson, is the last element that has been synthesized. The next two elements, elements 119 and 120, should form an 8s series and be an alkali and alkaline earth metal respectively. Beyond element 120, the superactinide series is expected to begin, when the 8s electrons and the filling of the 8p1/2, 7d3/2, 6f, and 5g subshells determine the chemistry of these elements. Complete and accurate CCSD calculations are not available for elements beyond 122 because of the extreme complexity of the situation: the 5g, 6f, and 7d orbitals should have about the same energy level, and in the region of element 160, the 9s, 8p3/2, and 9p1/2 orbitals should also be about equal in energy. This will cause the electron shells to mix so that the block concept no longer applies very well, and will also result in novel chemical properties that will make positioning some of these elements in a periodic table very difficult.[18]

Energy eigenvalues (in eV) for the outermost electrons of elements with Z = 100 through 172, predicted using Dirac–Fock calculations. The − and + signs refer to orbitals with decreased or increased azimuthal quantum number from spin–orbit splitting respectively: p− is p1/2, p+ is p3/2, d− is d3/2, d+ is d5/2, f− is f5/2, f+ is f7/2, g− is g7/2, and g+ is g9/2.[19]

The first two elements of period 8 will be ununennium and unbinilium, elements 119 and 120. Their electron configurations should have the 8s orbital being filled. This orbital is relativistically stabilized and contracted; thus, elements 119 and 120 should be more like rubidium and strontium than their immediate neighbours above, francium and radium. Another effect of the relativistic contraction of the 8s orbital is that the atomic radii of these two elements should be about the same as those of francium and radium. They should behave like normal alkali and alkaline earth metals (albeit less reactive than their immediate vertical neighbours), normally forming +1 and +2 oxidation states respectively, but the relativistic destabilization of the 7p3/2 subshell and the relatively low ionization energies of the 7p3/2 electrons should make higher oxidation states like +3 and +4 (respectively) possible as well.[4][18]

The superactinide series is expected to contain elements 121 to 157.[17] In the superactinide series, the 7d3/2, 8p1/2, 6f5/2 and 5g7/2 shells should all fill simultaneously.[19] This creates very complicated situations, so much so that complete and accurate CCSD calculations have been done only for elements 121 and 122.[18] The first superactinide, unbiunium (element 121), should be a congener of lanthanum and actinium and should have similar properties to them:[64] its main oxidation state should be +3, although the closeness of the valence subshells' energy levels may permit higher oxidation states, just as in elements 119 and 120.[18] Relativistic stabilization of the 8p subshell should result in a ground-state 8s28p1 valence electron configuration for element 121, in contrast to the ds2 configurations of lanthanum and actinium.[18] Its first ionization energy is predicted to be 429.4 kJ/mol, which would be lower than those of all known elements except for the alkali metalspotassium, rubidium, caesium, and francium: this value is even lower than that of the period 8 alkali metal ununennium (463 kJ/mol). Similarly, the next superactinide, unbibium (element 122), may be a congener of cerium and thorium, with a main oxidation state of +4, but would have a ground-state 7d18s28p1 valence electron configuration, unlike thorium's 6d27s2 configuration. Hence, its first ionization energy would be smaller than thorium's (Th: 6.3 eV; Ubb: 5.6 eV) because of the greater ease of ionizing unbibium's 8p1/2 electron than thorium's 6d electron.[18] The collapse of the 5g orbital itself is delayed until element 125; the electron configurations of the 119-electron isoelectronic series are expected to be [Og]8s1 for elements 119 through 122, [Og]6f1 for elements 123 and 124, and [Og]5g1 for element 125 onwards.[65]

In the first few superactinides, the binding energies of the added electrons are predicted to be small enough that they can lose all their valence electrons; for example, unbihexium (element 126) could easily form a +8 oxidation state, and even higher oxidation states for the next few elements may be possible. Unbihexium is also predicted to display a variety of other oxidation states: recent calculations have suggested a stable monofluoride UbhF may be possible, resulting from a bonding interaction between the 5g orbital on unbihexium and the 2p orbital on fluorine.[66] Other predicted oxidation states include +2, +4, and +6; +4 is expected to be the most usual oxidation state of unbihexium.[19] The superactinides from unbipentium (element 125) to unbiennium (element 129) are predicted to exhibit a +6 oxidation state and form hexafluorides, though UbpF6 and UbhF6 are predicted to be relatively weakly bound. Stable monofluorides of these elements may also be possible.[65] The bond dissociation energies are expected to greatly increase at element 127 and even more so at element 129. This suggests a shift from strong ionic character in fluorides of element 125 to more covalent character, involving the 8p orbital, in fluorides of element 129. The bonding in these superactinide hexafluorides is mostly between the highest 8p subshell of the superactinide and the 2p subshell of fluorine, unlike how uranium uses its 5f and 6d orbitals for bonding in uranium hexafluoride.[65]

Despite the ability of early superactinides to reach high oxidation states, it has been calculated that the 5g electrons will be most difficult to ionize; the Ubp6+ and Ubh7+ ions are expected to bear a 5g1 configuration, similar to the 5f1 configuration of the Np6+ ion.[14][65] Similar behavior is observed in the chemical inactivity of the 4f electrons in lanthanides; this is a consequence of the 5g orbitals being small and deeply buried in the electron cloud.[14] The presence of electrons in g-orbitals, which do not exist in the ground state electron configuration of any currently known element, should allow presently unknown hybrid orbitals to form and influence the chemistry of the superactinides in new ways, although the absence of g electrons in known elements makes predicting superactinide chemistry more difficult.[4]

In the later superactinides, the oxidation states should become lower. By element 132, the predominant most stable oxidation state will be only +6; this is further reduced to +3 and +4 by element 144, and at the end of the superactinide series it will be only +2 (and possibly even 0) because the 6f shell, which is being filled at that point, is deep inside the electron cloud and the 8s and 8p1/2 electrons are bound too strongly to be chemically active. The 5g shell should be filled at element 144 and the 6f shell at around element 154, and at this region of the superactinides the 8p1/2 electrons are bound so strongly that they are no longer active chemically, so that only a few electrons can participate in chemical reactions. Calculations by Fricke et al. predict that at element 154, the 6f shell is full and there are no d- or other electron wave functions outside the chemically inactive 8s and 8p1/2 shells. This may cause element 154 to be rather unreactive with noble gas-like properties.[4][18] Calculations by Pyykkö nonetheless expect that at element 155, the 6f shell is still chemically ionisable: Upp3+ should have a full 6f shell, and the fourth ionisation potential should be between those of terbium and dysprosium, both of which are known in the +4 state.[14]

Similarly to the lanthanide and actinide contractions, there should be a superactinide contraction in the superactinide series where the ionic radii of the superactinides are smaller than expected. In the lanthanides, the contraction is about 4.4 pm per element; in the actinides, it is about 3 pm per element. The contraction is larger in the lanthanides than in the actinides due to the greater localization of the 4f wave function as compared to the 5f wave function. Comparisons with the wave functions of the outer electrons of the lanthanides, actinides, and superactinides lead to a prediction of a contraction of about 2 pm per element in the superactinides; although this is smaller than the contractions in the lanthanides and actinides, its total effect is larger due to the fact that 32 electrons are filled in the deeply buried 5g and 6f shells, instead of just 14 electrons being filled in the 4f and 5f shells in the lanthanides and actinides respectively.[4]

Pekka Pyykkö divides these superactinides into three series: a 5g series (elements 121 to 138), an 8p1/2 series (elements 139 to 140), and a 6f series (elements 141 to 155), also noting that there would be a great deal of overlapping between energy levels and that the 6f, 7d, or 8p1/2 orbitals could well also be occupied in the early superactinide atoms or ions. He also expects that they would behave more like "superlanthanides", in the sense that the 5g electrons would mostly be chemically inactive, similarly to how only one or two 4f electrons in each lanthanide are ever ionized in chemical compounds. He also predicted that the possible oxidation states of the superactinides might rise very high in the 6f series, to values such as +12 in element 148.[14] Andrey Kul'sha has called the thirty-six elements 121 to 156 "ultransition" elements and has proposed to split them into two series of eighteen each, one from elements 121 to 138 and another from elements 139 to 156. The first would be analogous to the lanthanides, with oxidation states mainly ranging from +4 to +6, as the filling of the 5g shell dominates and neighbouring elements are very similar to each other, creating an analogy to uranium, neptunium, and plutonium. The second would be analogous to the actinides: at the beginning (around elements in the 140s) very high oxidation states would be expected as the 6f shell rises above the 7d one, but the typical oxidation states would lower and in elements in the 150s onwards the 8p1/2 electrons would stop being chemically active. Because the two rows are separated by the addition of a complete 5g18 subshell, they could be considered analogues of each other as well.[68]

As an example from the late superactinides, element 156 is expected to exhibit mainly the +2 oxidation state, on account of its electron configuration with easily removed 7d2 electrons over a stable [Og]5g186f148s28p21/2 core. It can thus be considered a heavier congener of nobelium, which likewise has a pair of easily removed 7s2 electrons over a stable [Rn]5f14 core, and is usually in the +2 state (strong oxidisers are required to obtain nobelium in the +3 state).[68] Its first ionization energy should be about 400 kJ/mol and its metallic radius should be about 170 picometers. It should be a very heavy metal with a density of around 26 g/cm3. Its relative atomic mass should be around 445 u.[4]

The transition metals in period 8 are expected to be elements 157 to 165 (or perhaps with element 121 replacing 157, similarly to the dispute on whether lanthanum or lutetium is better placed as the first 5d transition metal). To these, element 166 may be added to complete the 7d subshell, although like its lighter group 12 homologues, it is questionable if it would show transition metal character. Although the 8s and 8p1/2 electrons are bound so strongly in these elements that they should not be able to take part in any chemical reactions, the 9s and 9p1/2 levels are expected to be readily available for hybridization.[4][18] These 7d elements should be similar to the 4d elements yttrium through cadmium.[68] In particular, element 164 with a 7d109s0 electron configuration shows clear analogies with palladium with its 4d105s0 electron configuration.[19]

The noble metals of this series of transition metals are not expected to be as noble as their lighter homologues, due to the absence of an outer s shell for shielding and also because the 7d shell is strongly split into two subshells due to relativistic effects. This causes the first ionization energies of the 7d transition metals to be smaller than those of their lighter congeners.[4][18][19]

Calculations predict that the 7d electrons of element 164 (unhexquadium) should participate very readily in chemical reactions, so that unhexquadium should be able to show stable +6 and +4 oxidation states in addition to the normal +2 state in aqueous solutions with strong ligands. Unhexquadium should thus be able to form compounds like Uhq(CO)4, Uhq(PF3)4 (both tetrahedral like the corresponding palladium compounds), and Uhq(CN)2−2 (linear), which is very different behavior from that of lead, which unhexquadium would be a heavier homologue of if not for relativistic effects. Nevertheless, the divalent state would be the main one in aqueous solution (although the +4 and +6 states would be possible with stronger ligands), and unhexquadium(II) should behave more similarly to lead than unhexquadium(IV) and unhexquadium(VI).[18][19]

Unhexquadium is expected to be a soft Lewis acid and have Ahrlands softness parameter close to 4 eV. Unhexquadium should be at most moderately reactive, having a first ionization energy that should be around 685 kJ/mol, comparable to that of molybdenum.[4][19] Due to the lanthanide, actinide, and superactinide contractions, unhexquadium should have a metallic radius of only 158 pm, very close to that of the much lighter magnesium, despite its expected atomic weight of around 474 u which is about 19.5 times the atomic weight of magnesium.[4] This small radius and high weight cause it to be expected to have an extremely high density of around 46 g·cm−3, over twice that of osmium, currently the most dense element known, at 22.61 g·cm−3; unhexquadium should be the second most dense element in the first 172 elements in the periodic table, with only its neighbor unhextrium (element 163) being more dense (at 47 g·cm−3).[4] Metallic unhexquadium should have a very large cohesive energy (enthalpy of crystallization) due to its covalent bonds, most probably resulting in a high melting point. In the metallic state, unhexquadium should be quite noble and analogous to palladium and platinum. Fricke et al. suggested some formal similarities to oganesson, as both elements have closed-shell configurations and similar ionisation energies, although they note that while oganesson would be a very bad noble gas, unhexquadium would be a good noble metal.[19]

Theoretical interest in the chemistry of unhexquadium is largely motivated by theoretical predictions that it, especially the isotopes 472Uhq and 482Uhq (with 164 protons and 308 or 318 neutrons), would be at the center of a hypothetical second island of stability (the first being centered on copernicium, particularly the isotopes 291Cn, 293Cn, and 296Cn which are expected to have half-lives of centuries or millennia).[69][45][70][71]

Elements 165 (unhexpentium) and 166 (unhexhexium), the last two 7d metals, should behave similarly to alkali and alkaline earth metals when in the +1 and +2 oxidation states respectively. The 9s electrons should have ionization energies comparable to those of the 3s electrons of sodium and magnesium, due to relativistic effects causing the 9s electrons to be much more strongly bound than non-relativistic calculations would predict. Elements 165 and 166 should normally exhibit the +1 and +2 oxidation states respectively, although the ionization energies of the 7d electrons are low enough to allow higher oxidation states like +3 for element 165. The oxidation state +4 for element 166 is less likely, creating a situation similar to the lighter elements in groups 11 and 12 (particularly gold and mercury).[4][18] As with mercury but not copernicium, ionization of element 166 to Uhh2+ is expected to result in a 7d10 configuration corresponding to the loss of the s-electrons but not the d-electrons, making it more analogous to the lighter "less relativistic" group 12 elements zinc, cadmium, and mercury, which have essentially no transition-metal character.[14]

Some predicted properties of elements 156–166The metallic radii and densities are first approximations.[4][14][18]Most analogous group is given first, followed by other similar groups.[19]

The next six elements on the periodic table are expected to be the last main-group elements in their period,[14] and are likely to be similar to the 5p elements indium through xenon.[68] In elements 167 to 172, the 9p1/2 and 8p3/2 shells will be filled. Their energy eigenvalues are so close together that they behave as one combined p-subshell, similar to the non-relativistic 2p and 3p subshells. Thus, the inert pair effect does not occur and the most common oxidation states of elements 167 to 170 are expected to be +3, +4, +5, and +6 respectively. Element 171 (unseptunium) is expected to show some similarities to the halogens, showing various oxidation states ranging from −1 to +7, although its physical properties are expected to be closer to that of a metal. Its electron affinity is expected to be 3.0 eV, allowing it to form HUsu, analogous to a hydrogen halide. The Usu− ion is expected to be a soft base, comparable to iodide (I−). Element 172 (unseptbium) is expected to be a noble gas with chemical behaviour similar to that of xenon, as their ionization energies should be very similar (Xe, 1170.4 kJ/mol; Usb, 1090 kJ/mol). The only main difference between them is that element 172, unlike xenon, is expected to be a liquid or a solid at standard temperature and pressure due to its much higher atomic weight.[4] Unseptbium is expected to be a strong Lewis acid, forming fluorides and oxides, similarly to its lighter congener xenon.[19] Because of the analogy of elements 165–172 to periods 2 and 3, Fricke et al. considered them to form a ninth period of the periodic table, while the eighth period was taken by them to end at the noble metal element 164. This ninth period would be similar to the second and third period in that it is expected to have no transition metals.[19]

Some predicted properties of elements 167–172The metallic or covalent radii and densities are first approximations.[4][18]Most analogous group is given first, followed by other similar groups.[19]

Element 172, the last period 8 element, is expected to be the first noble gas after oganesson (the last period 7 element). Beyond it another long transition series like the superactinides should begin, filling at least the 6g, 7f, and 8d shells (with 10s, 10p1/2, and 6h11/2 too high in energy to contribute early in the series). These electrons would be very loosely bound, potentially rendering extremely high oxidation states reachable, though the electrons would become more tightly bound as the ionic charge rises.[19]

In element 173 (unsepttrium), the outermost electron would enter the 6g7/2 subshell. Because spin-orbit interactions would create a very large energy gap between the 8p3/2 and 6g7/2 subshells, this outermost electron is expected to be very loosely bound and very easily lost to form a Ust+ cation. As a result, element 173 is expected to behave chemically like an alkali metal, and one by far more reactive than even caesium (francium and element 119 being less reactive than caesium due to relativistic effects).[72][68]

Element 184 (unoctquadium) was significantly targeted in early predictions, as it was originally speculated that 184 would be a proton magic number: it is predicted to have an electron configuration of [Usb] 6g5 7f4 8d3, with at least the 7f and 8d electrons chemically active. Its chemical behaviour is expected to be similar to uranium and neptunium, as further ionisation past the +6 state (corresponding to removal of the 6g electrons) is likely to be unprofitable; the +4 state should be most common in aqueous solution, with +5 and +6 reachable in solid compounds.[4][19][73]

The number of physically possible elements is unknown. A low estimate is that the periodic table may end soon after the island of stability,[15] which is expected to center on Z = 126, as the extension of the periodic and nuclides tables is restricted by the proton and the neutron drip lines and stability toward alpha decay and spontaneous fission.[74] One calculation by Y. Gambhir et al., analyzing nuclear binding energy and stability in various decay channels, suggests a limit to the existence of bound nuclei at Z = 146.[75] Some, such as Walter Greiner, predicted that there may not be an end to the periodic table.[16] Other predictions of an end to the periodic table include Z = 128 (John Emsley) and Z = 155 (Albert Khazan).[11]

It is a "folk legend" among physicists that Richard Feynman suggested that neutral atoms could not exist for atomic numbers greater than Z = 137, on the grounds that the relativisticDirac equation predicts that the ground-state energy of the innermost electron in such an atom would be an imaginary number. Here, the number 137 arises as the inverse of the fine-structure constant. By this argument, neutral atoms cannot exist beyond untriseptium (alternatively called "feynmanium"), and therefore a periodic table of elements based on electron orbitals breaks down at this point. However, this argument presumes that the atomic nucleus is pointlike. A more accurate calculation must take into account the small, but nonzero, size of the nucleus, which is predicted to push the limit further to Z ≈ 173.[16]

where Z is the atomic number, and α is the fine structure constant, a measure of the strength of electromagnetic interactions.[76] Under this approximation, any element with an atomic number of greater than 137 would require 1s electrons to be traveling faster than c, the speed of light. Hence, the non-relativistic Bohr model is inaccurate when applied to such an element.

Energy eigenvalues for the 1s, 2s, 2p1/2 and 2p3/2 shells from solutions of the Dirac equation (taking into account the finite size of the nucleus) for Z = 135–175 (–·–), for the Thomas-Fermi potential (—) and for Z = 160–170 with the self-consistent potential (---).[4]

where m is the rest mass of the electron. For Z > 137, the wave function of the Dirac ground state is oscillatory, rather than bound, and there is no gap between the positive and negative energy spectra, as in the Klein paradox.[77] More accurate calculations taking into account the effects of the finite size of the nucleus indicate that the binding energy first exceeds 2mc2 for Z > Zcr ≈ 173. For Z > Zcr, if the innermost orbital (1s) is not filled, the electric field of the nucleus will pull an electron out of the vacuum, resulting in the spontaneous emission of a positron.[78][79] This diving of the 1s subshell into the negative continuum has often been taken to constitute an "end" to the periodic table, although more detailed treatments suggest a less bleak outcome.[14][16][80]

Atoms with atomic numbers above Zcr ≈ 173 have been termed supercritical atoms. Supercritical atoms cannot be totally ionised because their 1s subshell would be filled by spontaneous pair creation in which an electron-positron pair is created from the negative continuum, with the electron being bound and the positron escaping. However, the strong field around the atomic nucleus is restricted to a very small region of space, so that the Pauli exclusion principle forbids further spontaneous pair creation once the subshells that have dived into the negative continuum are filled. Elements 173–184 have been termed weakly supercritical atoms as for them only the 1s shell has dived into the negative continuum; the 2p1/2 shell is expected to join around element 185 and the 2s shell around element 245. Experiments have so far not succeeded in detecting spontaneous pair creation from assembling supercritical charges through the collision of heavy nuclei (e.g. colliding lead with uranium to momentarily give an effective Z of 174; uranium with uranium gives effective Z = 184 and uranium with californium gives effective Z = 190). As supercritical atoms are expected to pose no difficulties with their electronic structure, the end of the periodic table may be determined by nuclear instability rather than electron shell instabilities.[81]

Predicted half-lives (top) and decay modes (bottom) of superheavy nuclei. The line of synthesized proton-rich nuclei is expected to be broken soon after Z = 120, because of half-lives shorter than 1 microsecond from Z = 121, the increasing contribution of spontaneous fission instead of alpha decay from Z = 122 onward until it dominates from Z = 125, and the proton drip line around Z = 130. The white rings denote the expected location of the island of stability; the two squares outlined in white denote 291Cn and 293Cn, predicted to be the longest-lived nuclides on the island with half-lives of centuries or millennia.[47]

Taking nuclear deformation and relativistic effects into account, an analysis of single-particle levels predicts new magic numbers for superheavy nuclei at Z = 126, 138, 154, and 164 and N = 228, 308, and 318.[10][69] Therefore, in addition to the island of stability centered at 291Cn, 293Cn,[26] and 298Fl, further islands of stability may exist around the doubly magic 354126 as well as 472164 or 482164.[70][71] These nuclei are predicted to be beta-stable and decay by alpha emission or spontaneous fission with relatively long half-lives, and confer additional stability on neighboring N = 228 isotones and elements 152–168, respectively.[84] On the other hand, the same analysis suggests that proton shell closures may be relatively weak or even nonexistent in some cases such as 354126, meaning that such nuclei might not be doubly magic and stability will instead be primarily determined by strong neutron shell closures.[69] Additionally, due to the enormously greater forces of electromagnetic repulsion that must be overcome by the strong force at the second island (Z = 164),[85] it is possible that nuclei around this region only exist as resonances and cannot stay together for a meaningful amount of time. It is also possible that some of the superactinides between these series may not actually exist because they are too far from both islands,[85] in which case the periodic table might end around Z = 130.[19]

Beyond element 164, the fissility line defining the limit of stability with respect to spontaneous fission may converge with the neutron drip line, posing a limit to the existence of heavier elements.[84] Nevertheless, further magic numbers have been predicted at Z = 210, 274, and 354 and N = 308, 406, 524, 644, and 772,[86] with two beta-stable doubly magic nuclei found at 616210 and 798274; the same calculation method reproduced the predictions for 298Fl and 472164. (The doubly magic nuclei predicted for Z = 354 are beta-unstable, with 998354 being neutron-deficient and 1126354 being neutron-rich.) Although additional stability toward alpha decay and fission are predicted for 616210 and 798274, with half-lives up to hundreds of microseconds for 616210,[86] there will not exist islands of stability as significant as those predicted at Z = 114 and 164. As the existence of superheavy elements is very strongly dependent on stabilizing effects from closed shells, nuclear instability and fission will likely determine the end of the periodic table beyond these islands of stability.[19][75][84]

As the main island of stability is thought to lie around 291Cn and 293Cn, undiscovered elements beyond oganesson may be very unstable and undergo alpha decay or spontaneous fission in microseconds or less. The exact region in which half-lives exceed one microsecond is unknown, though various models suggest that isotopes of elements heavier than unbinilium that may be produced in fusion reactions with available targets and projectiles will have half-lives under one microsecond and therefore may not be detected.[47] It is consistently predicted that there will exist regions of stability at N = 184 and N = 228, and possibly also at Z ~ 124 and N ~ 198. These nuclei may have half-lives of a few seconds and undergo predominantly alpha decay and spontaneous fission, though minor beta-plus decay (or electron capture) branches may also exist.[87] Outside these regions of enhanced stability, fission barriers are expected to drop significantly due to loss of stabilization effects, resulting in fission half-lives below 10−18 seconds, especially in even-even nuclei for which hindrance is even lower due to nucleon pairing.[84] In general, alpha decay half-lives are expected to increase with neutron number, from nanoseconds in the most neutron-deficient isotopes to seconds closer to the beta-stability line.[36] For nuclei with only a few neutrons more than a magic number, binding energy substantially drops, resulting in a break in the trend and shorter half-lives.[36] The most neutron deficient isotopes of these elements may also be unbound and undergo proton emission. Cluster decay (heavy particle emission) has also been proposed as an alternative decay mode for some isotopes,[88] posing yet another hurdle to identification of these elements.

The following are the expected electron configurations of elements 118–173. Beyond element 123, no complete calculations are available and hence the data in this table must be taken as tentative.[19][72][89] In the case of element 123, and perhaps also heavier elements, several possible electron configurations are predicted to have very similar energy levels, such that it is very difficult to predict the ground state.[89]

^"Element 117 synthesized on Russian atom smasher". The Voice of Russia. 7 April 2010. Archived from the original on 2015-10-02. Retrieved 4 October 2015. The Dubna team are now upgrading their smasher ahead of attempts to synthesize Elements 119 and 120 in a series of experiments before 2012.Cite uses deprecated parameter |dead-url= (help)

^ ab"Scientists will begin experiments on the synthesis of element 119 in 2019". www.jinr.ru. JINR. 28 September 2016. Retrieved 31 March 2017. "The discovery of elements 115, 117 and 118 is an accomplished fact; they were placed in the periodic table, though still unnamed and will be confirmed only at the end of the year. The D.I.Mendeleev Periodic Table is not infinite. In 2019, scientists will begin the synthesis of elements 119 and 120 which are the first in the 8th period," said S.N. Dmitriev.